This invention relates to a variable-displacement compressor of a swash plate type known in the art.
Japanese Unexamined Utility Model Publication (JP-U) No. 19083/1988 discloses an example of such a compressor. The compressor includes a drive shaft continuously rotated while the compressor is driven. Hereinafter, such a compressor will be called a direct-coupled pulley type.
Referring to FIG. 1, description will be made as regards a conventional variable-displacement compressor 1 of the direct-coupled pulley type. The conventional variable-displacement compressor 1 comprises a housing 2 having, at an end thereof in a predetermined direction, a cylindrical portion 21a extending in the predetermined direction, a drive shaft 3 rotatably held to the housing 2 and extending within the cylindrical portion 21a to have a first and a second axial end opposite to each other, a compression mechanism connected to the first axial end of the shaft 3 and contained in the housing 1 for carrying out a predetermined compressing operation in the manner known in the art.
The conventional variable-displacement compressor 1 further comprises a pulley 4 rotatably supported on the cylindrical portion 21a to be coaxial with the shaft 3, and a rotation transmission plate 5 coupling the pulley 4 and the shaft 3 to each other to directly transmit a rotary torque of the pulley 4 to the shaft 3. The rotation transmission plate 5 has a torque limiting mechanism for limiting the rotary torque in the manner known in the art.
The conventional variable-displacement compressor is operable independent of an ambient temperature as far as a compressor drive source is operated. In the variable-displacement compressor of the swash plate type, the housing 2 defines a crankcase having a crankcase pressure which is controlled to regulate a cooling ability. When the ambient temperature is low, the cooling ability of a low level is sufficient. In this event, the crankcase pressure is increased to decrease the cooling ability.
When the cooling ability is decreased beyond a predetermined level, the crankcase pressure will dramatically increase. In this state, the shaft 3 is subjected to retracting force acting in a thrust direction, namely, the predetermined direction so that the shaft 3 will be retracted into the housing 2 in the manner known in the art. As illustrated in FIG. 1, the retracting force is received by a positioning nut 19 disposed at a center portion of a cylinder block 23 and a thrust bearing 20 facing the first axial end of the shaft 3 in the predetermined direction.
As described in the foregoing, the variable-displacement compressor of the direct-coupled pulley type is continuously operated independent of the ambient temperature. If the ambient temperature is kept low, the shaft is continuously subjected to the above-mentioned retracting force in the thrust direction. This means that the thrust bearing facing the first axial end of the shaft continuously receives the retracting force and often suffers a defect such as a seizure.